What is Huntington's Disease?

HD is a genetic disorder that is passed from one generation to the next. Each child of a parent with HD has a 50% chance of inheriting the gene that causes HD. Both males and females may have HD and may transmit the gene to sons and daughters. All ethnic groups are affected. The symptoms of HD usually appear during the late 30s to mid-40s, but sometimes become apparent in young children or the elderly. Symptoms vary widely, but three areas are commonly affected: personality, thinking and movement.

Emotional and behavioral changes such as depression, irritability, anxiety, aggressive outbursts, mood swings and social withdrawal can occur in people with HD. Thinking abilities such as short-term memory, organizational and coping skills may be impaired. Physical symptoms such as fidgeting, twitching and difficulties in coordination may appear, gradually leading to more obvious involuntary movements of the head, trunk, and limbs as the disorder progresses. The ability to walk and maintain balance may be affected, as can speech and swallowing. Symptoms typically become worse as time passes, so treatment is changed to try to respond to the patient's needs.

The Genetics of Huntington's Disease

http://www.gene.com/ae/ In each cell of your body you have 46 chromosomes which are arranged in 23 pairs. One chromosome from each pair comes from your mother and the other from your father. Chromosomes are made up of small units of genetic material called genes. The gene for HD is located on chromosome number 4.

Genes are made up of deoxyribonucleic acid, or DNA. DNA molecules consist of chains of four small elements called bases.

http://www.gene.com/ae/ There are four bases known as A (adenine), T (thymine), G (guanine), and C (cytosine). The order of the bases constitutes a code which determines the type of protein that the particular gene produces. Any change in the sequence of bases may cause a problem in the functioning of that protein.

The gene causing HD has been shown to have a region in which three bases (CAG) are repeated many times.

Individuals who have repeat numbers up to 35 are not expected to develop HD. In the gene causing HD, this region contains approximately 40 or more such repeats. Some individuals have a gene that has between 36 and 39 repeats. This "intermediate" situation is more difficult to interpret and may require further analysis by the laboratory to determine if the individual is likely to develop HD.
Because our genes come in pairs, each person will have a certain number of repeats in one gene inherited from one parent and another number of repeats in the other gene from the other parent. The repeat sizes can be the same (such as 18 in both genes) or different (such as 18 repeats in one gene and 42 in the other). Each parent passes one of his/her genes on to the child. Analyzing the number of repeats in this region on each gene can be done on very small samples of DNA, which can be obtained from blood samples and from most tissues.